I replaced the stock seatpost on my wife's '08 Madison with a Kalloy Uno 250mm x 27.2mm seatpost (I just went with the specs listed on Schwinn's site too), it was a little snug like the stock seatpost but it fit fine. I even compared the width to the stock seatpost before installing it and it looked like it matched.
I'll be replacing the seatpost yet again with a 250mm x 27.2mm Thomson Elite seatpost that I won in an eBay auction, hope that fits too
Edit: My wife's bike also came with Jalco rims and chrome Truvativ cranks/silver chainring.